Various coupling systems or coupling apparatuses are in practical use for horizontally and vertically positioning and locking on the table of a machine tool, a pallet (a movable article) to which a work piece to be machined is attached. In such coupling systems, the pallet positioning accuracy largely affects the machining accuracy. Recently, the pallet positioning error is required to be as small as approximately 4 μm.
U.S. Pat. No. 4,855,558 discloses a machining tool electrode holder wherein as a means for positioning the electrode holder for mounting the electrodes relative to the chuck with precision, the electrode holder has four slots in the shape of a cross and the chuck has four rails (projections) to be engaged with the four slots so that the four rails are engaged with the four slots for positioning them. Recesses are formed on either wall near the slot opening and a pair of elastically deformable tongue-shaped parts is formed on the walls. Elastic deformation of the tongue-shaped parts serves to absorb manufacturing errors of the slots or rails.
U.S. Pat. No. 5,634,757 discloses a coupling apparatus for tools or work pieces wherein the upper holder has four projections (inverted trapezoid in cross-section) in the shape of a cross on the bottom surface, a work piece or tool to be coupled to the holder has four recesses (inverted trapezoid in cross-section) with which the four projections are engaged on the top surface. The projections have flat slots on the wall near the tip. The holder is brought near to the work piece so that the four projections are engaged with the four recesses, the coupling bolts are tightened, and the walls are elastically deformed through the slots, whereby the bottom surface of the holder abuts against the top surface of the work piece or tool. In this way, the work piece or tool is positioned horizontally and vertically (in the X, Y, Z directions) relative to the holder and coupled to it by the bolts. The slot serves to tolerate elastic deformation of the wall near the projection, thereby absorbing manufacturing errors of the recesses or projections.
U.S. Pat. No. 6,152,436 discloses a machining tool coupling apparatus wherein a square base has on the top surface four cams arranged in the shape of a cross in the middle of the four sides of a square framing wall and the pallet has on the bottom surface four engaging grooves to be engaged with the four cams. The four cams are engaged with the four engaging grooves so that the pallet is positioned horizontally (in the X, Y directions) relative to the base. For the vertical positioning (in the Z direction), the base has four seats in the four corners of the base framing wall. The bottom surface of the pallet rests on the seats to position it in the Z direction.
The cams on the framing wall have a slit near the bottom, through which the elastic wall near the bottom of the cam can be elastically deformed. The ball-type clamping mechanism equipped in the center of the base absorbs manufacturing errors of the cams, engaging grooves, or seats through elastic deformation of the elastic wall when the pallet is clamped.    Patent Document 1: U.S. Pat. No. 4,855,558;    Patent Document 2: U.S. Pat. No. 5,634,757; and    Patent Document 3: U.S. Pat. No. 6,152,436.